Light emitting device having singlet and triplet compounds with different emission colors

ABSTRACT

There is provided a light emitting device which enables a color display with good color balance. A triplet compound is used for a light emitting layer of an EL element that emits red color, and a singlet compound is used for a light emitting layer of an EL element that emits green color and a light emitting layer of an EL element that emits blue color. Thus, an operation voltage of the EL element emitting red color may be made the same as the EL element emitting green color and the EL element emitting blue color. Accordingly, the color display with good color balance can be realized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/049,423, filed Mar. 17, 2008, now U.S. Pat. No. 7,915,808, which is acontinuation of U.S. application Ser. No. 11/105,414, filed Apr. 14,2005, now U.S. Pat. No. 7,400,087, which is a continuation of U.S.application Ser. No. 09/871,805, filed Jun. 4, 2001, now U.S. Pat. No.7,339,317, which claims the benefit of a foreign priority applicationfiled in Japan as Serial No. 2000-168325 on Jun. 5, 2000, all of whichare incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device (hereinafter, referred to as alight emitting device) having an element (hereinafter, referred to as alight emitting element) sandwiching a luminous material betweenelectrodes. In particular, the present invention relates to a lightemitting device having a light emitting element (hereinafter, referredto as an EL element) using an organic compound from which EL (electroluminescence) is obtained as a luminous material Note that, an organicEL display and an organic light emitting diode (OLED) are included in alight emitting device of the present invention.

Further, the luminous materials that may be used for the presentinvention include is all the luminous materials that luminesce(phosphorescence and/or fluorescence) via a singlet excitation or atriplet excitation, or via both the excitations.

2. Description of the Related Art

In recent years, the development of an EL element with an organic ELfilm as a light emitting layer is progressing, and the EL elements usingvarious organic EL films have been proposed. Also, experiments torealize a flat panel display using a light emitting device with such anEL element as a light emitting element have been conducted.

As a light emitting device using an EL element, there are known apassive matrix type and an active matrix type. The passive matrix typelight emitting device is provided with stripe shaped anode and cathodewhich are orthogonal to each other, and uses an EL element having astructure sandwiching an EL film between the anode and the cathode.Further, the active matrix type light emitting device is provided with athin film transistor (hereinafter, referred to as a TFT) for each pixel,and is of a method for controlling current flowing in the EL element bythe TFT connected to one of an anode and a cathode of an EL element.

Further, there are proposed various methods for color display of a lightemitting device using EL elements. There is known a method forperforming color display by mixing luminescence of colors, with threepixels, a pixel which emits red light, a pixel which emits green lightand a pixel which emits blue light as one unit.

Such a method is attracting attention since a bright color display iseasily obtained. However, since the EL elements which emit light ofrespective colors use different organic EL films as light emittinglayers, the brightness characteristic of the light emitting layer (therelationship between operation voltage and the brightness) differs. As aresult, the operation voltage necessary in obtaining a desiredbrightness differs for each EL element, and further the reliability(life) of the light emitting layer differs for each EL element.

From the above, it is feared that not only there will be an increase inthe kinds of power sources necessary for the light emitting device, butalso that a shift in the color balance due to difference in the life(deterioration rate) of the EL element will be generated.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problem, andtherefore an object of the present invention is to provide a techniquefor keeping operation voltages of an EL element with red light emission,an EL element with green light emission and an EL element with bluelight emission at a constant level, when a light emitting device is madeto perform color display. Another object of the present invention is toprovide a light emitting device that can perform color display with goodcolor balance.

A further object of the present invention is to provide an electricappliance which has a display portion with high image quality, byemploying the light emitting device which may perform color display withgood color balance as its display portion.

In the present invention, there is a feature in that as a light emittinglayer, an organic compound which emits light by a singlet exciton(singlet) (hereinafter, referred to as a singlet compound), and anorganic compound which emits light by a triplet exciton (triplet)(hereinafter, referred to as a triplet compound) are used together. Notethat, in this specification, the singlet compound refers to a compoundwhich emits light via only a singlet excitation, and the tripletcompound refers to a compound which emits light via only a tripletexcitation.

As to a triplet compound, the organic compounds disclosed in thefollowing articles may be given as typical materials.

-   (1) T. Tsutsui, C. Adachi, S. Saito, Photochemical Processes in    Organized Molecular Systems, ed. K. Honda, (Elsevier Sci. Pub.,    Tokyo, 1991) p. 437.-   (2) M. A. Baldo, D. F. O□Brien, Y. You, A. Shoustikov, S.    Sibley, M. E. Thompson, S. R. Forrest, Nature 395 (1998) p. 151.

In these articles are disclosed the organic compounds shown by thefollowing formulas.

-   (3) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.    Forrest, Appl. Phys. Lett., 75 (1999) p. 4.-   (4) T. Tsutsui, M. J. Yang, M. Yahiro, K. Nakamura, T. Watanabe, T.    Tsuji, Y. Fukuda, T. Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38    (12B) (1999) L1502.

Further, the present inventors consider that not only the luminousmaterials disclosed in the above articles, but also the luminousmaterials represented by the following molecular formulas (specificallya metal complex or an organic compound) may be used.

In the above molecular formulas, M represents an element belonging toGroups 8 to 10 of the periodic table. In the above articles, platinumand iridium are used. Further, the present inventors consider that sincenickel, cobalt and palladium are cheaper than platinum and iridium, theyare more preferable in reducing the manufacturing cost of the lightemitting device. Especially, since nickel can easily form a complex, theproductivity is high and therefore preferable.

The triplet compound has higher luminous efficiency than the singletcompound, and the operation voltage (a voltage necessary for making anEL element emit light) may be decreased in obtaining the same lightemitting brightness. This embodiment makes use of this feature.

FIG. 1 shows a cross sectional structure of a pixel portion of the lightemitting device of the present invention. In FIG. 1, reference numeral10 indicates an insulator, reference numeral 11 indicates a currentcontrolling TFT, reference numeral 12 indicates a pixel electrode(anode), reference numeral 13 indicates an insulating film with anopening on the pixel electrode (hereinafter, referred to as a bank),reference numeral 14 indicates a hole injecting layer, reference numeral15 indicates a light emitting layer which emits red light, referencenumeral 16 indicates a light emitting layer which emits green light,reference numeral 17 indicates a light emitting layer which emits bluelight, reference numeral 18 indicates an electron transporting layer,and reference numeral 19 indicates a cathode.

Note that, FIG. 1 shows an example of using a bottom gate type TFT(specifically an inverted stagger type TFT) as a current controllingTFT, but a top gate type TFT may be used. Further, known organiccompounds or inorganic compounds may be used for the hole injectinglayer 14, the light emitting layer 15 which emits red to light, thelight emitting layer 16 which emits green light, the light emittinglayer 17 which emits blue light, or the electron transporting layer 18,respectively.

In this embodiment, the triplet compound is used as the light emittinglayer 15 which emits red light, and the singlet compound is used as thelight emitting layer 16 which emits green light and the light emittinglayer 17 which emits blue light. That is, an EL element using a tripletcompound is used as an EL element which emits red light, and an ETelement using a singlet compound is used as an EL element which emitsgreen or blue light.

When using a low molecular organic compound as a light emitting layer,at present the life of a light emitting layer which emits red light isshorter than that of a light emitting layer which emits other coloredlight. This is because the luminous efficiency is lower than that ofother colors, and in order to obtain the same light emitting brightnessas other colors, the operation voltage has to be set higher and progressof deterioration for that amount is fast.

However, in the present invention, since the triplet compound with highluminous efficiency is used as the light emitting layer 15 which emitsred light, the operation voltages may be made the same whilst the sameluminous brightness as the light emitting layer 16 which emits greenlight and the light emitting layer 17 which emits blue light may beobtained. Accordingly, the deterioration of the light emitting layer 15which emits red light does not progress significantly, and color displaymay be performed without causing a problem of shift in color balance, orthe like. Further, suppressing the operation voltage as low ispreferable also from the point of view that the margin of the withstandpressure of the transistor may be set low Note that, in the presentinvention, an example of using the triplet compound as the lightemitting layer 15 which emits red light is shown, but further by usingthe triplet compound for the light emitting layer 16 which emits greenlight and the light emitting layer 17 which emits blue light, theoperation voltages of the respective EL elements may be made the same.

Next, a circuit structure of a pixel portion of the light emittingdevice of the present invention is shown in FIGS. 2A and 2B. Note that,here a pixel including an EL element which emits red light (pixel (red))20 a, a pixel including an EL element which emits green light (pixel(green)) 20 b, and a pixel including an EL element which emits bluelight (pixel (blue)) 20 c are shown, and all the pixels have the samecircuit structure.

In FIG. 2A, reference numerals 21 indicates a gate wiring, referencenumerals 22 a to 22 c indicate source wirings (data wiring), andreference numerals 23 a to 23 c indicate current supply lines. Thecurrent supply lines 23 a to 23 c are wirings which determine theoperation voltages of the EL elements, and the same voltage is appliedto any of the pixels, the pixel 20 a which emits red light, the pixel 20b which emits green light and the pixel 20 c which emits blue light.Accordingly, the line width (thickness) of the wiring may all have thesame design.

Further, reference numerals 74 a to 24 c indicate switching TFTs (TFTsfor controlling the signal to be input to the gate of the currentcontrolling TFT), and here the switching TFTs are formed of n-channelTFTs. Note that, here a structure where two channel forming regionsexist between a source region and a drain region is illustrated, butthere may be one or more than two channel forming regions.

Further, reference numerals 25 a to 25 c indicate current controllingTFTs (TFTs for controlling the current flowing in the EL element), andgate electrodes of the current controlling TFTs 25 a to 25 c arerespectively connected to the switching TFTs 24 a to 24 c, sourceregions of the current controlling TFTs 25 a to 25 c are respectivelyconnected to the current supply lines 23 a to 23 c, drain regions of thecurrent controlling TFTs 25 a to 25 c are respectively connected to ELelements 26 a to 26 c. Note that, reference numerals 27 a to 27 cindicate capacitors which maintain the voltage applied to the gateelectrodes of the respective current controlling 11-Ts 25 a to 25 c.However, the capacitor 27 a to 27 c may be omitted. Note that, FIG. 2Ashows an example where the switching TFTs 24 a to 24 c formed ofn-channel TFTs and the current controlling TFTs 25 a to 25 c formed ofp-channel TFTs are provided. However, as shown in FIG. 2B, for pixel(red) 30 a, pixel (green) 30 b and pixel (blue) 30 c, switching TFTs 28a to 28 c formed of p-channel TFTs and current controlling TFTs 29 a to29 c formed of n-channel TFTs may also be provided, respectively.

Further, FIGS. 2A and 2B show an example where two TFTs are provided inone pixel, but the number of TFTs may be three or more (typically 3 to6). In such a case, the n-channel TFTs and the p-channel TFTs may becombined in any way to be provided.

In FIGS. 2A and 2B, the EL element 26 a is an EL element emitting redcolor, and a triplet compound is used as a light emitting layer.Further, the EL element 26 b is an EL element which emits green light,the EL element 26 c is an EL element which emits blue light, and bothuse a singlet compound as the light emitting layer.

As described above, by using the triplet compound and the singletcompound properly, the operation voltages of the EL element which emitsred light, the EL element which emits green light and the EL elementwhich emits blue light may all be the same (10 V or less, preferably 3to 10 V). Accordingly, it is possible to suppress the shift in colorbalance due to the difference in the life of the EL element, and thepower source necessary for the light emitting device may be unified at 3V or 5 V. Thus, there is an advantage that the circuit design becomeseasier.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a cross sectional structure of a pixelportion of a light emitting device of the present invention;

FIGS. 2A and 2B are diagrams showing circuit structures of the pixelportion of the light emitting device of the present invention;

FIG. 3 is a diagram showing a cross sectional structure of a pixelportion of a light emitting device of Embodiment 1;

FIG. 4 is a diagram showing a cross sectional structure of a pixelportion of a light emitting device of Embodiment 2;

FIG. 5 is a diagram showing a circuit structure of a pixel portion of alight emitting device of Embodiment 3;

FIG. 6 is a diagram showing a cross sectional structure of the pixelportion of the light emitting device of Embodiment 3;

FIG. 7 is a diagram showing a circuit structure of a pixel portion of alight emitting device of Embodiment 4;

FIG. 8 is a diagram showing a cross sectional structure of the pixelportion of the light emitting device of Embodiment 4;

FIG. 9 is a diagram showing a cross sectional structure of a pixelportion of a light emitting device of Embodiment 5;

FIGS. 10A and 10B are diagrams showing a structure of a driving circuitbuilt-in light emitting device of Embodiment 6;

FIGS. 11A and 11B are diagrams showing a structure of a light emittingdevice externally mounted with a driver circuit of Embodiment 7;

FIGS. 12A and 12B are diagrams showing a structure of a light emittingdevice externally mounted with a controller of Embodiment 7;

FIGS. 13A to 13F are diagrams showing specific examples of electricapparatus of Embodiment 8; and

FIGS. 14A and 14B are diagrams showing other specific examples ofelectric apparatus of Embodiment 8.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described in detailreferring to the following embodiments.

Embodiment 1

In this embodiment, as a light emitting device of the present invention,an example of a light emitting device is shown, in which the device hasa pixel portion 151 and a driver circuit 150 which drives the pixelportion on the same insulator (however, in a state before sealing). Notethat, a CMOS circuit as a basic unit is shown for the driver circuit150, and one pixel is shown for the pixel portion 151. However, inactuality the structure of the pixel portion is made by gathering aplurality of pixels as shown in FIG. 1.

In FIG. 3, reference numeral 100 indicates an insulator (including aninsulating substrate, an insulating film or a substrate having aninsulating film on the surface), with an n-channel TFT 201, a p-channelTFT 202, a switching TFT 203 formed of an n-channel TFT, and a currentcontrolling TFT 204 formed of a p-channel TFT formed thereon. At thistime, the circuit structure of the pixel portion is the structure shownin FIG. 2A. Further, in this embodiment, the TFTs are all formed ofinverted stagger type TFTs.

First, the structures of the n-channel TFT 201 and the p-channel TFT 202are described.

In the n-channel TFT 201, reference numeral 101 indicates a gateelectrode, reference numeral 102 indicates a gate insulating film,reference numeral 103 indicates a source region, reference numeral 104indicates a drain region, reference numerals 105 a and 105 b indicateLDD (light doped drain) regions, reference numeral 106 indicates achannel forming region, reference numeral 107 indicates a channelprotective film, reference numeral 108 indicates a first interlayerinsulating film, reference numeral 109 indicates a source wiring, andreference numeral 110 indicates a drain wiring.

In the p-channel TFT 202, reference numeral 111 indicates a gateelectrode, reference numeral 102 indicates the gate insulating film,reference numeral 112 indicates a source region, reference numeral 113indicates a drain region, reference numeral 114 indicates a channelforming region, reference numeral 115 indicates a channel protectivefilm, reference numeral 108 indicates the first interlayer insulatingfilm, reference numeral 116 indicates a source wiring and referencenumeral 110 indicates the drain wiring. The drain wiring 110 is a wiringwhich is common with the n-channel TFT 201.

The switching TFT 203 has a structure having two channel forming regionsbetween a source region and a drain region. It may be easily understoodwith reference to the description of the structure of the n-channel TFT201, and thus, the explanation is omitted. Further, with reference tothe description of the structure of the p-channel TFT 202, the currentcontrolling TFT 204 may be easily understood, and therefore theexplanation is omitted.

Then, a second interlayer insulating film (leveling film) 119 isprovided covering the n-channel TFT 201, the p-channel TFT 202, theswitching TFT 203 and the current controlling TFT 204.

Note that, before the second interlayer insulating film 119 is provided,a contact hole 118 is provided in the first interlayer insulating film108 over a drain region 117 of the current controlling TFT 204. This isfor making the etching process easy when forming a contact hole in thesecond interlayer insulating film 119.

Further, in the second interlayer insulating film 119, a contact hole isformed to reach the drain region 117, and a pixel electrode 120connected to the drain region 117 is provided. The pixel electrode 120functions as an anode of the EL element, and a conductive film with alarge work function, typically an oxide conductive film a is used. Asthe oxide conductive film, indium oxide, tin oxide, zinc oxide or acompound thereof may be used.

Next, reference numeral 121 indicates a bank, which is an insulatingfilm provided to cover an end portion of the pixel electrode 120. Thebank 121 may be formed of an insulating film or a resin film includingsilicon. When using a resin film, carbon particles or metal particlesare added so that the resistance of the resin film is made 1_(—)10⁶ to1_(—)10¹² Ωm (preferably 1_(—)10⁸ to 1_(—)10¹⁰ Ωm). Thus, dielectricbreakdown at the time of film formation may be suppressed.

Next, reference numeral 122 indicates an EL layer. Note that, in thisspecification, a laminate body with a hole injecting layer, a holetransporting layer, a hole preventing layer, an electron transportinglayer, an electron injecting layer or an electron preventing layercombined with a light emitting layer is referred to as an EL layer. Itis a feature of the present invention that the singlet compound and thetriplet compound are used together as the light emitting layer.

Note that, in this embodiment, the triplet compound is used as anorganic compound used in the EL element which emits red light, and thesinglet compound is used as an organic compound used in the EL elementwhich emits green light and the EL element which emits blue light. Atthis time as the triplet compound, the organic compounds mentioned abovemay be used, and as the singlet compound, an Alq₃ (aluminumquinolinolate complex) with a fluorescent pigment coevaporated may beused.

Next, reference numeral 123 indicates a cathode of an EL element, whichuses a conductive film with a small work function. As the conductivefilm with a small work function, a conductive film containing an elementbelonging to Group 1 or 2 of the periodic table may be used. In thisembodiment, a conductive film formed of a compound of lithium andaluminum may be used.

Note that, a laminating body 205 formed of the pixel electrode (anode)120, the EL layer 122 and the cathode 123 is the EL element. The lightemission generated by the EL element 205 is irradiated to the side ofthe insulator 100 (the direction of the arrow in the figure). Further,when using the p-channel TFT for the current controlling TFT 204 as inthis embodiment, the drain region 117 of the current controlling TFT 204is preferably connected with an anode of the EL element 205.

Note that, although not shown here, after the formation of the cathode123, it is effective to provide a passivation film to completely coverthe EL element 205. The passivation film is formed of an insulating filmincluding a carbon film, a silicon nitride film or a silicon nitrideoxide film, and may be formed of a single layer or a lamination layer ofthe insulating film.

At this time, it is preferable to use a film with good coverage as thepassivation film, and it is effective to use a carbon film, especially aDLC (diamond like carbon) film. The DLC film may be formed in atemperature range between a room temperature and 100_C, and therefore,the DLC film may be easily formed above the EL layer 122 with low heatresistance. Further, the blocking effect of the DLC film to oxygen ishigh, and oxidation of the EL layer 122 may be suppressed. Therefore,the problem of oxidation of the EL layer 122 during the sealing processto be subsequently performed may be prevented.

In the light emitting device of the present invention having the pixelportion and the driver circuit with the above structures, the singletcompound and the triplet compound for the EL elements are used properly,so that the operation voltages of the EL elements may be made the same,and a good color display with excellent color balance may be performed.

Further, the operation voltages of the EL elements may all be made 10Vor less (typically 3 to 10V), so that an advantage that the circuitdesign becomes easy is obtained.

Embodiment 2

In this embodiment, as a light emitting device of the present invention,an example of a light emitting device is shown, in which the device hasa pixel portion and a driver circuit which drives the pixel portion onthe same insulator (however, in a state before sealing). Note that, aCMOS circuit as a basic unit is shown for the driver circuit 250, andone pixel is shown for the pixel portion 251. However, in practice thestructure of the pixel portion is made as shown in FIG. 1. Further, theportions with the same symbols attached as FIG. 3 may refer to thedescription of Embodiment 1.

In FIG. 4, reference numeral 100 indicates the insulator, with then-channel TFT 201, the p-channel TFT 202, a switching TFT 206 formed ofa p-channel TFT, and a current controlling TFT 207 formed of ann-channel TFT formed thereon. At this time, the circuit structure of thepixel portion 251 is the structure shown in FIG. 2B. Further, in thisembodiment, the TFTs are all formed of inverted stagger type TFTs.

The descriptions of the n-channel TFT 201 and the p-channel TFT 202 maybe referred to Embodiment 1 and thus the descriptions will be omitted.Further, the switching TFT 206 has a structure having two channelforming regions between a source region and a drain region, but may beeasily understood by referring to the description of the structure ofthe p-channel TFT 202. Thus, the description is omitted. In addition,the current controlling TFT 207 may be easily understood by referring tothe description of the structure of the n-channel TFT 201, and thus thedescription is omitted.

In the case of this embodiment, the structure of an EL element differsfrom that of Embodiment 1. A drain region 301 of the current controllingTFT 207 is connected with a pixel electrode 302. The pixel electrode 302is an electrode which functions as a cathode of an EL element 208, andis formed using a conductive film containing an element belonging toGroup 1 or 2 of the periodic table. In this embodiment, a conductivefilm formed of a compound of lithium and aluminum is used.

Further, the EL element 208 is formed of the pixel electrode (cathode)302, an EL layer 303 and an anode 304. Note that, in this embodiment, atriplet compound is used as an organic compound used for an EL elementwhich emits red light, and a singlet compound is used as an organiccompound used for an EL element which emits green light and an ELelement which emits blue light. At this time, as the triplet compound,the organic compounds mentioned above may be used, and as the singletcompound, an Alq₃ (aluminum quinolinolate complex) with a fluorescentpigment coevaporated may be used.

Further, in this embodiment, as the anode 304, an oxide conductive filmwith gallium oxide added to zinc oxide is used. Since the oxideconductive film transmits visible light, the light generated in the ELelement 208 is irradiated toward the top surface of the anode 304 (inthe direction of the arrow in the figure). Note that, when using then-channel TFT for the current controlling TFT 207 as in this embodiment,it is preferable that the drain region 301 of the current controllingTFT 207 is connected to the cathode of the EL element 208.

Note that, although not shown here, after the formation of the anode304, it is effective to provide a passivation film to completely coverthe EL element 208. The passivation film is formed of an insulating filmincluding a carbon Elm, a silicon nitride film or a silicon nitrideoxide film, and may be formed of a single layer or a lamination layer ofthe insulating film.

In the light emitting device of the present invention having the pixelportion and the driver circuit with the above structures, the singletcompound and the triplet so compound for the EL elements are usedproperly, so that the operation voltages of the EL elements may be madethe same, and a good color display with excellent color balance may beperformed.

Further, the operation voltages of the EL elements may all be made 10Vor less (typically 3 to 10V), so that an advantage that the circuitdesign becomes easy is is obtained.

Note that, the structure of this embodiment may be implemented incombination with the structure in Embodiment 1.

Embodiment 3

In this embodiment, as a light emitting device of the present invention,a case where a pixel portion and a driver circuit are all formed ofn-channel TFTs is described. Note that, the circuit structure of a pixelof this embodiment is as shown in FIG. 5. Further, the description inFIGS. 2A and 2B may be referred to for portions with the same symbolsattached as in FIGS. 2A and 2B.

As shown in FIG. 5, the switching TFTs 24 a to 24 c and currentcontrolling TFTs 36 a to 36 c formed respectively for a pixel (red) 33a, a pixel (green) 35 b and a pixel (blue) 35 c, are all formed ofn-channel TFTs.

Here the cross sectional structure of the light emitting device of thisembodiment (however, in a state before sealing) is shown in FIG. 6. Notethat, a CMOS circuit as a basic unit is shown for the driver circuit350, and one pixel is shown for the pixel portion 331. However, inactuality the structure of the pixel portion 351 is as shown in FIG. 1.Further, for the portions where the same symbols as in FIGS. 3 and 4 areattached, the description in Embodiments 1 or 2 may be referred to.

In FIG. 6, reference numeral 100 indicates the insulator, with then-channel TFT 201, an n-channel TFT 209, the switching TFT 203 formed ofan n-channel TFT, and the current controlling TFT 207 formed of ann-channel TFT formed thereon. At this time, the circuit structure of thepixel portion is the structure shown in FIG. 5.

Further, in this embodiment, the TFTs are all formed of inverted staggertype TFTs. At this time, the n-channel TFTs may all be enhancement typeTFTs, or may all be depression type TFTs. Of course, both may be usedproperly in combination. The enhancement type or the depression type,may be selected by adding n-type or p-type impurities into the channelforming region.

The n-channel TFT 201 and the n-channel TFT 209 have the same structure.The explanation may be referred to Embodiment 1, and therefore isomitted. Further, the switching TFT 203 has a structure where twochannel forming regions exist between a source region and a drainregion, and it may be easily understood by referring to the descriptionof the structure of the n-channel TFT 201. Therefore, the description isomitted. Further, the current controlling TFT 207 may easily beunderstood by referring to the description of the structure of then-channel TFT 201, and therefore the description is omitted.

In the case of this embodiment, the structure of the EL element is thesame as Embodiment 2. That is, in this embodiment, since an n-channelTFT is used for the current controlling TFT 207, it is preferable thatthe cathode 302 of the EL element 208 is connected to the drain region301 of the current controlling TFT 207. Embodiment 2 may be referred forthe description related to the EL element.

Note that, although not shown here, after the formation of the anode304, it is effective to provide a passivation film to completely coverthe EL element 208. The passivation film is formed of an insulating filmincluding a carbon film, a silicon nitride film or a silicon nitrideoxide film, and may be formed of a single layer or a lamination layer ofthe insulating film.

In the light emitting device of the present invention having the pixelportion 351 and the driver circuit 350 with the above structures, thesinglet compound and the triplet compound for the EL elements are usedproperly, so that the operation voltages of the EL elements may be madethe same, and a good color display with excellent color balance may beperformed. Further, since all the operation voltages of the EL elementsmay be made 10 V or less (typically 3 to 10 V), there is obtained anadvantage that the circuit design is easily made.

Furthermore, according to the structure of this embodiment, aphotolithography process for forming a p-channel TFT can be omitted.Thus, the manufacturing process can be simplified.

Note that the structure of this embodiment can be implemented incombination with the structure described in Embodiment 1 or Embodiment2.

Embodiment 4

In this embodiment, the case where a pixel portion and a driver circuitare all formed of p-channel TFTs in a light emitting device of thepresent invention is explained. Note that the circuit configuration of apixel in this embodiment is as shown in FIG. 7. In addition, theexplanation for FIGS. 2A and 2B may be referred as to the portionsindicated by the same reference symbols as those in FIGS. 2A and 2B.

As shown in FIG. 7, switching TFTs 51 a to 51 c and current controllingTFTs 52 a to 52 c, which are respectively provided in a pixel (red) 50a, a pixel (green) 30 b, and a pixel (blue) 50 c, are all formed ofp-channel TFTs.

FIG. 8 shows a cross-sectional structure of the light emitting device inthis embodiment (in the state before sealing). Note that a CMOS circuitas a basic unit is shown for the driver circuit and one pixel is shownfor the pixel portion. However, in actuality, the structure of the pixelportion is as shown in FIG. 1. Further, the explanation in Embodiment 1or Embodiment 2 may be referred to as to the portions indicated by thesame reference symbols as those in FIG. 3 or FIG. 4. In FIG. 8,reference numeral 100 indicates the insulator, with a p-channel TFT 210,the p-channel TFT 202, the switching TFT 206 formed of a p-channel TFT,and the current controlling TFT 204 formed of a p-channel TFT formedthereon. At this time, the circuit configuration of the pixel portion451 is as shown in FIG. 7.

Further, in this embodiment, the TFTs are all formed of p-channelinverted stagger type TFTs. At this time, all the p-channel TFTs may beenhancement type TFTs and may be depression type TFTs. Of course, thep-channel TFTs may be formed of using both types of the TFTs properly incombination. The enhancement type or the depression type may be selectedby adding an n-type impurity or a p-type impurity in a channel formingregion.

The p-channel TFT 210 and the p-channel TFT 202 have the same structure,and the explanation therefor is omitted since Embodiment 1 may bereferred thereto. Further, the switching TFT 206 has the structure inwhich two channel forming regions are interposed between a source regionand a drain region. Since the switching TFT 206 can be easily understoodwith reference to the explanation for the structure of the p-channel TFT202, the explanation therefor is omitted. In addition, since the currentcontrolling TFT 204 can be easily understood with reference to theexplanation for the p-channel TFT 202, the explanation therefor isomitted.

In case of this embodiment, the structure of an EL element is the sameas that in Embodiment 1. That is, in this embodiment, a p-channel TFT isused for the current controlling TFT 204, and thus, it is preferablethat the drain region 117 of the current controlling TFT 204 isconnected with the anode 120 of the EL element 205. The explanation forthe EL element may be referred to Embodiment 1.

Note that although not shown, it is effective to provide a passivationfilm so as to entirely cover the EL element 205 after the formation ofthe cathode 123. A single layer or a lamination layer of the insulatingfilm comprising a carbon film, a silicon nitride film, or a siliconoxide nitride film is used for the passivation film.

In a light emitting device of the present invention, which includes thepixel portion 451 and the driver circuit 450 having the abovestructures, the operation voltages of the EL elements may be made thesame since a singlet compound and a triplet compound are properly usedfor the EL elements. Thus, a good color display with excellent colorbalance may be realized.

Further, since all the operation voltages of the EL elements may be made10 V or less (typically 3 to 10 V), there is obtained an advantage thatthe circuit design is easily made.

Furthermore, according to the configuration of this embodiment, aphotolithography process for forming an n-channel TFT can be omitted.Thus, the manufacturing process can be simplified.

Note that the configuration of this embodiment can be implemented incombination with the configuration described in Embodiment 1 orEmbodiment 2.

Embodiment 5

An example of using the top gate TFT (specifically planar TFT) as aswitching TFT and the current control TFT is shown in this embodiment.

FIG. 9 shows a cross sectional structure of the pixel portion in theactive matrix type light emitting device of this embodiment. In FIG. 9,reference numeral 910 shows an insulator, reference numeral 911 shows acurrent control TFT, reference numeral 912 shows a pixel electrode(anode), reference numeral 913 shows a bank, reference numeral 914 showsa known hole injecting layer, reference numeral 915 shows a lightemitting layer which emits red color, reference numeral 916 shows alight emitting layer which emits green color, reference numeral 917shows a light emitting layer which emits blue color, reference numeral918 shows a known electron transporting layer, and reference numeral 919shows a cathode.

Here in this embodiment, a triplet compound is used as a light emittinglayer 915 which emits red color, and a singlet compound is used as alight emitting layer 916 which emits green color and a light emittinglayer 917 which emits blue color. That is, an EL element using a singletcompound is an EL element which emits green color or blue color, and anEL element using the above-mentioned triplet compound is an EL elementwhich emits red color.

However, in this embodiment since a triplet compound with high luminousefficiency is used as the light emitting layer 915 which emits redcolor, the same light emitting brightness as the light emitting layer916 which emits green color and the light emitting layer 917 which emitsblue color may be obtained while the operation voltage is made the same.Accordingly, the deterioration of the light emitting layer 915 whichemits red color does not progress significantly, and color display maybe performed without causing a problem such as color shift. Further,suppression of the operation voltage is preferable considering that themargin of the resistance of the transistor may be set low.

Note that in this embodiment an example of using a triplet compound asthe light emitting layer 915 which emits red color is shown, and atriplet compound may be used as the light emitting layer 916 which emitsgreen color or the light emitting layer 917 which emits blue color.

A circuit structure in the case this embodiment is implemented is shownin FIG. 2. Note that, the structures of this embodiment may beimplemented in combination with any of the structures of Embodiments 1to 4.

Embodiment 6

Further, the light emitting device of the embodiment after the seal (orencapsulation) step for protecting the EL element is performed will bedescribed with reference to FIGS. 10A and 10B. An example of the sealstep of structure shown in Embodiment 1 (FIG. 3). Further the sealstructure of this embodiment may be implemented in combination with anyof the structures of Embodiments 1 to 5. Note that, reference numeralsused in FIG. 3 is cited as needed.

FIG. 10A is a top view showing a state where steps up to sealing of anEL element are performed, and FIG. 10B is a cross sectional view of FIG.10A taken along with the line A-A′. Reference numeral 501 of a portionshown by a dotted line designates a pixel portion; 502, a source sidedriving circuit; and 503, a gate side driving circuit. Reference numeral504 designates a cover member; 505, a first seal member; and 506, asecond seal member.

Note that, reference numeral 508 designates a wiring line fortransmitting signals inputted to the source side driving circuit 502 andthe gate side driving circuit 503, which receives a video signal and aclock signal from an FPC (Flexible Print Circuit) as an external inputterminal. Note that, although only the FPC is shown here, a print wiringboard (PWB) may be attached to the FPC.

Next, a cross sectional structure will be described with reference toFIG. 10B. A pixel portion 501 and a source side driving circuit 502 areformed on the top of an insulator 100, and the pixel portion 501 isformed of a plurality of pixels including a current controlling TFT 204and a pixel electrode 120 electrically connected to its drain. Thesource side driving circuit 502 is formed by using a CMOS circuit inwhich an n-channel TFT 201 and a p-channel TFT 202 are combined. Notethat, a polarizing plate (typically, a circular polarizing plate) may bebonded to the insulator 501.

The pixel electrode 120 functions as an anode of the EL element. Banks121 are formed at both ends of the pixel electrode 120, and an EL layer122 and a cathode 123 of the EL element are formed on the pixelelectrode 120. The cathode 123 functions also as a wiring line common toall pixels, and is electrically connected to the FPC 508 through theconnection wiring line 507. Further, all elements included in the pixelportion 501 and the source side driving circuit 502 are covered with apassivation film 509.

A cover member 504 is bonded with a first seal member 505. A spacer maybe provided to secure an interval between the cover member 504 and theEL element. A space 510 is formed inside of the first seal member 505.It is desirable that the first seal member 505 is a material which wateror oxygen does not permeate. Further, it is effective to provide amaterial having a moisture absorption effect or a material having anoxidation preventing effect in the inside of the space 510.

Note that, it is appropriate that carbon films (specifically,diamond-like carbon films) 511 a and 511 b as protection films areformed to a thickness of 2 to 30 nm on the front surface and the rearsurface of the cover member 504. The carbon film like this has a role toprevent the infiltration of oxygen and water and to mechanically protectthe surface of the cover member 504.

Besides, after the cover member 504 is adhered, a second seal member 506is provided so as to cover the exposed surface of the first seal member505. The second seal member 506 can be made of the same material as thefirst seal member 505.

By encapsulating the EL element in the structure as described above, theEL element can be completely cut off from the outside, and it ispossible to prevent a material accelerating deterioration due tooxidation of the EL layer such as moisture or oxygen, from infiltratingfrom the outside. Accordingly, the light emitting device having highreliability can be obtained.

Note that, as shown in FIGS. 10A and 10B, the light emitting device inwhich the pixel portion and the driving circuit are provided on the samesubstrate and the FPC is attached, is especially called a drivingcircuit built-in light emitting device in the present specification.

Embodiment 7

In Embodiment 6, the driving circuit built-in light emitting deviceshown in FIGS. 10A and 10B is given as the example in which a pixelportion and a driver circuit are integrally formed on the sameinsulator. However, it is possible to provide the driver circuit as anexternal IC (integrated circuit). In this case, the structure is asshown in FIG. 11A.

In the module shown in FIG. 11A, a substrate 60 (including a pixelportion 61 and wirings 62 a and 62 b), on which the pixel portionincluding a TFT and an EL element is formed, is provided with an FPC 63,and a printed wiring board 64 is attached to the substrate 60 throughthe FPC 63. Here, FIG. 11B is a functional block diagram of the printedwiring board 64.

As shown in FIG. 11B, an IC functioning as at least I/O ports (alsoreferred to as input or output portions) 65 and 68, a source drivercircuit 66, and a gate driver circuit 67 is provided inside the printedwiring board 64.

As described above, a module in which an FPC is attached to a substrateon which a pixel portion is formed, and a printed wiring board having afunction of a driver circuit is attached to the substrate through theFPC is referred to as a light emitting module with an external drivercircuit particularly in this specification.

Further, in the module shown in FIG. 12A, an FPC 74 is attached to adriving circuit built-in light emitting device 70 (including a pixelportion 71, a source driver circuit 72, a gate driver circuit 73, andwirings 72 a and 73 a), and a printed wiring is board 75 is attached tothe light emitting device 70 through the FPC 74. Here, FIG. 12B is afunctional block diagram of the printed wiring board 75.

As shown in FIG. 12B, an IC functioning as at least I/O ports 76 and 79,and a control portion 77 is provided inside the printed wiring board 75.Note that a memory portion 78 is provided here, but it is notnecessarily required. Further, the control portion 77 has a function forcontrolling a driver circuit, correction of image data and the like.

As described above, a module in which a printed wiring board having afunction of a controller is attached to a driving circuit built-in lightemitting device in which a Pixel portion and a driver circuit are formedon a surface of a substrate is referred to as a light emitting modulewith an external controller particularly in this specification.

Embodiment 8

The light-emitting device (including the module of which state is shownin Embodiment 7) formed by implementing this invention may be used as adisplay portion of various electrical apparatuses. As electricalapparatuses of this invention, there are such as a video camera, adigital camera, a goggle type display (head mounted display), anavigation system, a audio equipment, a note type personal computer, agame apparatus, a portable information terminal (such as a mobilecomputer, a portable telephone, a portable game apparatus or anelectronic book), and an image playback device with a recording medium.Specific examples of the electronic equipment are shown in FIGS. 13A-13Band 14A-14B.

FIG. 13A shows an EL display and includes a casing 2001, a supportingbase 2002 and a display portion 2003. The light-emitting device of thisinvention may be used for the display portion 2003. When using the ELlight-emitting device in the display portion 2003, it is a self-lightemitting type so that a back light is not necessary and the displayportion may be made thin.

FIG. 13B shows a video camera, which contains a main body 2101, adisplay portion 2102, a sound input portion 2103, operation switches2104, a battery 2103, and an image receiving portion 2106. Thelight-emitting device and the liquid crystal display device of thisinvention can be applied to the display portion 2102.

FIG. 13C shows a digital camera, which contains a main body 2201, adisplay is portion 2202, a eye contact portion 2203, and operationswitches 2204. The light emitting-device of this invention can beapplied to the display portion 2202.

FIG. 13D shows an image playback device equipped with a recording medium(specifically, a DVD playback device), which contains a main body 2301,a recording medium (such as a CD, LD or DVD) 2302, operation switches2303, a display portion (a) 2304, a display portion (b) 2305 and thelike. The display portion (a) is mainly used for displaying imageinformation. The display portion (b) 2303 is mainly used for displayingcharacter information. The light-emitting device of this invention canbe applied to the display portion (a) and the display portion (b). Notethat, the image playback device equipped with the recording mediumincludes devices such as CD playback device, and game machines.

FIG. 13E shows a portable (mobile) computer, which contains a main body2401, a display portion 2402, an image receiving portion 2403, operationswitches 2404 and a memory slot 2405. The light-emitting device of thisinvention can be applied to the display portion 2402. This portablecomputer may record information to a recording medium that hasaccumulated flash memory or involatile memory, and playback suchinformation.

FIG. 13F shows a personal computer, which contains a main body 2501, acasing 2502, a display portion 2503, and a keyboard 2504. Thelight-emitting device of this invention can be applied to the displayportion 2503.

The above electronic apparatuses more often display information sentthrough electron communication circuits such as the Internet or the CATV(cable television), and especially image information display isincreasing. When using the light-emitting device having the EL elementin the display portion, since the response speed of the EL element isextremely fast, it becomes possible to display pictures without delay.

Further, since the light emitting portion of the light-emitting deviceconsumes power, it is preferable to display information so that thelight emitting portion is as small as possible. Therefore, when usingthe light-emitting device in the portable information terminal,especially in the display portion where character information is mainlyshown in a cellular phone or an audio equipment, it is preferable todrive so that the character information is formed of a light emittingportion with the non-light emitting portion as a background.

Here, FIG. 14A shows a portable telephone, and reference numeral 2601shows a portion (operation portion) which performs key operation, andreference numeral 2602 shows a portion which performs informationdisplay (information display portion), and the operation portion 2601and the information display portion 2602 are connected by the connectingportion 2603. Further, the operation portion 2601 is provided with asound input portion 2604, operation switches 2605, and the informationdisplay potion 2602 is provided with a sound output portion 2606, adisplay portion 2607.

The light-emitting device of this invention may be used as the displayportion 2607. Note that, when using the light-emitting device to thedisplay portion 2607, the consumption power of the portable telephonemay be suppressed by displaying white letters in the background of theblack color.

In the case of the portable telephone shown in FIG. 14A, thelight-emitting device used in the display portion 2604 is incorporatedwith a sensor (a CMOS sensor) by a CMOS circuit, and may be used as anauthentication system terminal for authenticating the user by readingthe fingerprints or the hand print of the user. Further, light emissionmay be performed by taking into consideration the brightness(illumination) of outside and making information display at a contrastthat is already set.

Further, the low power consumption may be attained by decreasing thebrightness when using the operating switch 2605 and increasing thebrightness when the use of the operation switch is finished. Further,the brightness of the display portion 2604 is increased when a call isreceived, and low power consumption is attained by decreasing thebrightness during a telephone conversation. Further, when using thetelephone continuously, by making it have a function so that display isturned off by time control unless it is reset, low power consumption isrealized. It should be noted that this control may be operated by hand.

Further, FIG. 14B shows a car mounted type audio, which contains acasing 2701, a display portion 2702, and operation switches 2703 and2704. The light-emitting device this invention can be applied to thedisplay portion 2702. Further, in this embodiment, a car mounted audio(car audio) is shown, but it may be used in a fixed type audio (audiocomponent). Note that, when using a light-emitting device in the displayportion 2704, by displaying white characters in a black background,power consumption may be suppressed.

Further, electrical apparatuses shown above are incorporated with alight sensor in the light-emitting device which are used in the displayportion, and it is possible to provide means to detect the brightness ofthe environment of use. When using the light-emitting device in thedisplay portion, it is may have a function that modulates thelight-emission brightness according to the brightness of the environmentof use.

Specifically, this is implemented by providing an image sensor (surfaceshape, linear or a dotted sensor) formed by a CMOS circuit on thelight-emitting device using the display portion, and providing a CCD(charge coupled device) on the main body or the casing. The user mayrecognize the image or the character information without trouble if abrightness of a contrast ratio of 100 to 150 may be maintained ascompared to the brightness of the environment of use. Namely, in thecase the environment of use is dark, it is possible to suppress theconsumption power by suppressing the brightness of the image.

As in the above, the applicable range of this invention is extremelywide, and may be used for various electrical equipment. Further, theelectrical apparatuses of this embodiment may use the light-emittingdevice and module containing any of the structures of Embodiments 1 to7.

By implementing the present invention, the operation voltages of the ELelement which emits red light, the EL element which emits green light,the EL element which emits blue light may be made the same, and a lightemitting device which may perform a color display with good colorbalance may be provided.

Further, by using a light emitting device which may perform colordisplay with a good color balance in the display portion, an electricappliance having a good quality display portion may be provided.

1. A light emitting device comprising: a first light emitting layer; asecond light emitting layer; a third light emitting layer; and anelectrode over the first light emitting layer, the second light emittinglayer and the third light emitting layer, wherein at least one of thefirst light emitting layer, the second light emitting layer and thethird light emitting layer comprises a singlet compound, wherein atleast the other one of the first light emitting layer, the second lightemitting layer and the third light emitting layer comprises a tripletcompound, and wherein colors of light emitted from the first lightemitting layer, the second light emitting layer and the third lightemitting layer are different from each other.
 2. The light emittingdevice according to claim 1, wherein the first light emitting layer, thesecond light emitting layer and the third light emitting layer are notoverlapped with each other.
 3. The light emitting device according toclaim 1, wherein the electrode has a light transmitting property.
 4. Alight emitting device comprising: a first electrode; a first carrierinjecting layer over the first electrode; a first light emitting layerover the first carrier injecting layer; a second light emitting layerover the first carrier injecting layer; a third light emitting layerover the first carrier injecting layer; a second carrier injecting layerover the first light emitting layer, the second light emitting layer andthe third light emitting layer; and a second electrode over the secondcarrier injecting layer, wherein at least one of the first lightemitting layer, the second light emitting layer and the third lightemitting layer comprises a singlet compound, wherein at least the otherone of the first light emitting layer, the second light emitting layerand the third light emitting layer comprises a triplet compound, andwherein colors of light emitted from the first light emitting layer, thesecond light emitting layer and the third light emitting layer aredifferent from each other.
 5. The light emitting device according toclaim 4, wherein the first light emitting layer, the second lightemitting layer and the third light emitting layer are not overlappedwith each other.
 6. The light emitting device according to claim 4,wherein the first electrode has a light transmitting property.
 7. Thelight emitting device according to claim 4, wherein the second electrodehas a light transmitting property.
 8. The light emitting deviceaccording to claim 4, wherein the first carrier injecting layer is oneof a hole injecting layer and an electron injecting layer.
 9. The lightemitting device according to claim 8, wherein the second carrierinjecting layer is the other of the hole injecting layer and theelectron injecting layer.
 10. A light emitting device comprising: atransistor; an insulating film over the transistor; a first electrodeover the insulating film, the first electrode electrically connected tothe transistor; a first light emitting layer over the first electrode; asecond light emitting layer over the insulating film; a third lightemitting layer over the insulating film; and a second electrode over thefirst light emitting layer, the second light emitting layer and thethird light emitting layer, wherein at least one of the first lightemitting layer, the second light emitting layer and the third lightemitting layer comprises a singlet compound, wherein at least the otherone of the first light emitting layer, the second light emitting layerand the third light emitting layer comprises a triplet compound, andwherein colors of light emitted from the first light emitting layer, thesecond light emitting layer and the third light emitting layer aredifferent from each other.
 11. The light emitting device according toclaim 10, wherein the first light emitting layer, the second lightemitting layer and the third light emitting layer are not overlappedwith each other.
 12. The light emitting device according to claim 10,wherein the first electrode has a light transmitting property.
 13. Thelight emitting device according to claim 10, wherein the secondelectrode has a light transmitting property.
 14. The light emittingdevice according to claim 10, further comprising a second transistor anda third transistor, wherein the second light emitting layer is locatedover the second transistor; and wherein the third light emitting layeris located over the third transistor.
 15. A light emitting devicecomprising: a transistor; an insulating film over the transistor; afirst electrode over the insulating film, the first electrodeelectrically connected to the transistor; a first carrier injectinglayer over the first electrode; a first light emitting layer over thefirst carrier injecting layer; a second light emitting layer over thefirst carrier injecting layer; a third light emitting layer over thefirst carrier injecting layer; a second carrier injecting layer over thefirst light emitting layer, the second light emitting layer and thethird light emitting layer; and a second electrode over the secondcarrier injecting layer, wherein at least one of the first lightemitting layer, the second light emitting layer and the third lightemitting layer comprises a singlet compound, wherein at least the otherone of the first light emitting layer, the second light emitting layerand the third light emitting layer comprises a triplet compound, andwherein colors of light emitted from the first light emitting layer, thesecond light emitting layer and the third light emitting layer aredifferent from each other.
 16. The light emitting device according toclaim 15, wherein the first light emitting layer, the second lightemitting layer and the third light emitting layer are not overlappedwith each other.
 17. The light emitting device according to claim 15,wherein the first electrode has a light transmitting property.
 18. Thelight emitting device according to claim 15, wherein the secondelectrode has a light transmitting property.
 19. The light emittingdevice according to claim 15, wherein the first carrier injecting layeris one of a hole injecting layer and an electron injecting layer. 20.The light emitting device according to claim 19, wherein the secondcarrier injecting layer is the other of the hole injecting layer and theelectron injecting layer.